In addition to publishing updated tumor rates in control Fischer 344 rats and B6C3F1 mice, we compared the results of a number of chemicals evaluated both in NTP rodent carcinogenicity studies and in genetically altered mice. Preliminary results indicated that genetically altered mice have the potential to serve as more rapid and less expensive test systems to identify carcinogens. Twelve chemicals in the NTP database were identified that produced nasal cavity tumors. We found that most of the nasal carcinogens (1) produced tumor increases in both sexes, (2) produced tumors at other sites as well, (3) had significantly reduced survival at doses that were carcinogenic, and (4) were genotoxic. Finally, we found a striking inverse correlation between the incidences of pituitary gland tumors and testis adenoma in control male F344 rats. In dermal and inhalation studies, the control group incidences of both tumor types average approximately 65%. In contrast, in feeding and gavage studies, control groups consistently have lower incidences of pituitary gland neoplasms (averaging 32%), and higher incidences of testis tumors (averaging nearly 90%). One hypothesis that could account for this inverse correlation is stress associated with individual housing, a protocol that is used in dermal and inhalation studies, but not in feeding and gavage studies.